Easy way, get Partition Magic (with boot magic) and it will be a child play to set up dual or multi-boot. You do not even need 2nd HDD. It really a magic program. You can get it "new" 2nd hand from Amazon for £22
Hard way: You can also do it by editing boot sector etc, you need an expert to guide you for this. Not recommended.
Both above ways are just "soft" way because only need software.
Hardware way: get two HDD removeable house (you can get t from Mapline under £20), so you can upplug and plug either HDD easily.